1. Contribution from the Commissariat à l'Energie Atomique, MDN/SPSMS/DRFMC, CEN-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France, Department of Chemistry, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-Ku, Tokyo 153-8902, Japan, Institut Laue-Langevin, Av. des Martyrs, BP 156, 38042 Grenoble Cedex 9, France, and Commissariat à l'Energie Atomique, SCIB/DRFMC, CEN-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9